Method and apparatus for detecting heat



23, .v. K. ZWO-RYKIN El AL 2,159,755

I METHOD AND APPARATUS FOR DETECTING HEAT 'Filed May '7, 1936 l l l w l l l l Zsnventols 3 VA flD/fif/A? KZhokYK/A (Ittorneg electrode to the next adjacent electrode, thus Patented May 23, 1939 PATENT OFFICE 2,159,755 METHOD- AND APPARA JS FOR. DETECTING Vladimir K. Zworykin, Philadelphia, Pa., and,

Louis Malter, Oaklyn, N. J., assignors to Radio Gorporation of America, a corporation'of Delaware "Application May 7, 1936, Serial No. 78,344

2 Claims.

This invention relates to methods of, and apparatus for, detecting and measuring heat and particularly to electrical, means for measuring degrees of heat within andabove those indicated by the mercurial thermometer.

The invention contemplates and its construction provides an electron multiplier including a primary electron source, or cathode, which emits electrons when subjected to radiant. heat. The

emission of I primary-electrons may be purely thermionic or it may be brought about by utilizfing the infra-red rays in the heat. -,The pril mar-y stream of electrons whether produced by the infra-red rays or thermionically is caused to impinge upon thelmultiplying electrodes of an electron multiplier whereby the number of elec- :trons is augmented and the output of the multiplier raised to a level suitable for operating an indicator, measuringdevice, or relay.

The multiplier may conveniently be of the general type disclosed in French Patent 582,428 or of the type disclosed in the copending application of Louis Malter, Serial No. 4,049, filed J anuary 30, 1935 and assigned to Radio Corporation of America. In the above-identified copending application there is disclosed a multi-stageelectron multiplier constituted by a plurality of pairs t of opposed electrodes axially disposed in an evacuated containerwith their faces lying. in para a may be designatedrespectively, .upper or "accelerating Preferably the faces of the multiplying elecallel planes- For convenience the electrodes and lower or multiplying.

trodes are specially treated as by photosensitiza- 5 tion to enhance, their ability to emit secondary electrons and a primary electron source photoelectric or thermionic in character is provided.

Injaddition electro-magneticmeans are provided forthe purpose of causing the 'electrodesto describe approximately trochoidal paths from each permitting the use 'of. high accelerating potentials on the upper electrodes without the disadvantages resulting from the space charges entrons.

greater than unity. For example, a ratio of three ysix, or more secondary electrons to one v fimpingingelectron is readily. obtainable with multiplying electrodes in cascade, for example, an amplification of the original or primary electron current equivalent to the amplification per electrode raised to the nth power. A millionfold amplification is readily obtained in a single device.

The novel features characteristic of the present invention are set forth with particularity in the appended claims. The invention itself, howi ever, both as to its organization and method of operation will best be understood by reference to the following description taken in connection with the accompanying drawing in which Figure 1 is a view in perspective of a heat-indicator embodied in an electric discharge device of the type described in the Malter application, portions of the container wall and the associated magnetic field structure being broken away to more clearly illustrate the disposition and mounting of the electrodes;

Fig.2 is a view in perspective of another form of radiant-heat responsive cathode; and

, Fig. 3 is a diagrammatic view of a device, connected in circuit, and which employs an auxiliary heater for the cathode.

Referring now to Fig. 1 of the drawing, an electron multiplier embodying the invention is constituted by a cylindrical evacuated container T of glass, Pyrex or other suitable material wherein is disposed a plurality of electron emissive lowerelectrodes designated l, 3, 5, l and 9, respectively, lying in the same plane andspaced apart along thelong axis of the container and a plurality of upper or accelerating electrodes 2, 4, 6, 8 and I 0 disposed in a plane parallel to and spaced from the plane in which lie the first-mentioned electrodes.

The upperand lower electrodes are grouped, vertically in pairs. All of the electrodes have substantially the same length-breadth dimensions and they are spaced apart substantially container. Such configuration is desirable in that it permits accurate electron-path-control by means of a single magnetic field.

Each electrode is provided with an external lead (1" individual thereto. If desired the electrodes of each set (i. e., upper and lower) may be constructed as elements of a sub-assembly and each introduced into the tube as a unit as taught by the earlier Malter disclosure, or they may be arranged as elements in a unitary supporting assembly as disclosed: in copending application to Ernest Massa andLouis Malter,

Serial No. 33,996 filed July 31, 1935, and assigned to Radio Corporation of America.

An output electrode I2 is mounted in one end of the tube, preferably fairly close to the sets of electrodes and in a plane transverse to the tube axis. The output electrodeis provided with a terminal connection I211 which extends through the wall of the container to the outside thereof.

The upper electrodes are preferably made of molybdenum, tantalum, nickel or any other metal which is easily de-gassed and not easily oxidized. The accelerating electrode 2 which is paired with the primary-electron emitter or cath-. ode i is suitably orificed asat 2b to permit pas.- sage of heat rays.

The lower electrodes are preferably made of silver.. With the exception of the cathode i they may be of substantial and uniform thickness. The cathode l is the heat responsive primary-electron emitting electrode and is formed of a very thin sheet, say of the order of .0002" of silver foil, conveniently supported or backed by a strip of mica lb or other rigid poor heat conducting substance. It may, however, be constituted by a sheet of mica 2 lb having deposited thereon, as by spraying, a mere film 2 I of silver, asindicated in Fig. 2. Surfaces of both types emit thermionic electrons at comparatively low temperatures. With either construction the heat conduction to'the-supports is negligible, the only heat lost being due to radiaion.

In manufacturing the device of Fig. 1 the electrodes are first suitably mounted and the tube T is then heated and evacuated. After evacuation, oxygen is introduced into the container at a pressure in the neighborhood of 1 mm. of'mercury. One of the upper or accelerating electrodes is then made substantially 500 volts positive with respect to its corresponding lower electrode to cause a glow discharge which oxidizes the surface of the lower electrode. The oxidation iscarr'ied on until the lower surface acquires preferably a bluish green tinge. Each pair of electrodes is treated in the same manner and the process is continued until all of the lower electrodes, including the cathode, have acquired an oxide surface. r

After the lower electrodes are oxidized the residual oxygen is pumped out of the container and an alkali metal is ,distilledintoit. For this purpose caesium ispreferred, sodium, rubidium or potassium may, however, be employed. The tube is next baked for about ten minutes at a temperature of 210 C.; which causes the alkali metal to combine with the silver oxide, thus giving rise to a highly sensitive electron-emissive surface. During the baking step the excess caesium is pumped out of the device.

During the heating process just mentioned any caesium or other alkali metal which is deposited upon the upper electrodes or upon the container walls is driven off and whatever caesium is not removed by the pumping may be taken up if desired by a small amount of lead oxide which, though not shown in the drawing, may be introduced into the container at the time the electrodes are mounted therein. The lead oxide forms a relatively stable compound with the excess caesium and prevents it from being redeposited on the inner walls of the container where it would provide leakage paths upon the electrodes.

Any convenient means may be utilized for establishing a magnetic field parallel to the electrode surfaces such, for example, as the device M. Preferably this device M is constituted by an U-shaped element of magnetically permeable material on which is mounted an energizing coil 21 and to each upstanding portion of which is a fixed plate 29 also of permeable material. The tube is disposed between these plates in such position that a substantially uniform magnetic field is set. up parallel to the opposed surfaces of the sets of electrodes. Any convenient source of uni-directional potential may be used to supply field current to the coil 21. Obviously a permanent magnet may be substituted for the electromagnet shown, or the tube may be disposed within a coil of wire carrying an electric current.

The cathodes l and 2| of Figs. 1 and 2 do not begin to emit copiously until the temperature is raised to a certain point. Beyond this point the thermionic emission increases very rapidly with temperature.

Fig. 3 shows another embodiment of the invention and includes an auxiliary heater 3la. placed close ,to the cathode 3| so as to raise the temperature of the cathode to the point at which the thermionic emission sets in. The heat from the external source is permitted to raise the temperature of the cathode so that the heat brought from the external focusing surface S serves to raise the temperature only above this biaspoint instead of from room temperature only. The sensitivity of the device is thus enhanced by the additionof this auxiliary heater. The cathode and heater assembly may be constituted by an orificedsurface 3Ib which may be rectangular or annular in shape. Thesilver foil 3| constituting the emissive surface extends over the orifice and is anchored by any convenient means to this surface 3 lb. The auxiliary heater 31a is enclosed within a tubular shield 3 lo which may be accommodated in a dependent portion 3ld, of the tube.

It is necessary to maintain each of the lower multiplying electrodes positive with respect to the primary electron source and with respect to any electrode between it and the said source. For this purpose (referring to Fig. 3) the cathode 3| may be connected to the negative terminal of a source of unidirectional potential, exemplified in the drawing by battery B and resistor R, and the output electrode 42 may be connected to the positive terminal of the source through an output device 44. The first multiplying electrode 33 may be connected to a positive point 330. on the resistor R and each of the remaining multiplying electrodes 35 31, 39 connected to successively more positive points 350., 31a, 39a on the resistor.

The accelerating electrodes may each be maintained at the same potential as the next succeeding lower multiplying electrode and for this connecting leads 1 maybe provided within the tube. The accelerating electrode 40 is connected to a point 40a more positive than point 39a and the output or. collector electrode 42 is connected to a point 42a of still higher potential.

If heat rays, such, for example, as emanate from a furnace are focused as by a reflector S or by any othersuitable means, upon the cathode 3i, thermionic electrons will be emitted proporiii) tionally thereto. These thermionic-electrons will be accelerated toward the upper electrode 32 directly above the cathode 3| because of the electrostatic field and, if no magnetic field were present, they would impinge thereon. However, the magnetic field (not shown in Fig. 3), which is parallel to theplanes of the electrodes, subjects the electrons, moving toward the accelerating electrode, to a force component at a right angle to their instantaneous direction of travel. If the electric and magnetic fields are adjusted to proper values, the electrons will describe trochoidal paths and will strike the first multiplying electrode 33.

Electrode 33 is maintained at a potential positive with respect to the cathode 3| and the thermionic-electrons striking it will cause the emission of secondary electrons. The secondary electrons are in turn accelerated toward the second upper electrode 34, but, by reason of the magnetic field, they will be diverted and be focused upon the third lower electrode 35. Here again a multiplication, by reason of secondary emission, is secured and this process is repeated in the remaining stages 31 and 39, until the amplified stream of secondary electrons is collected by the output electrode 42 and the resultant current caused to actuate a meter, relay or the like exemplified in the drawing by device 44 included between the output electrode 42 and the positive terminal 420. of the potential source.

If a meter is employed it may be calibrated in degrees of temperature as indicated at Ma. If a suificient number of multiplying stages are employed to provide suificient power output the device may be utilized to directly actuate a motor driven fuel feed or other thermal control forming part of the system with which the device of the invention is associated.

What is claimed is:

l. A heat responsive device comprising an evacuated envelope containing a radiant-heat responsive electron-emissive cathode constituted essentially of silver foil of a thickness of the order of .0002 of an inch, means for multiplying electrons from said cathode, and a collector electrode upon which said multiplied electrons impinge.

2. A heat responsive device comprising an evacuated envelope containing a radiant-heat responsive electron-emissive cathode comprising a mica base having an evaporated silver film thereon, means for multiplying electrons from said cathode, and a collector electrode upon which said multiplied electrons impinge.

VLADIMIR K. ZWORYKIN. LOUIS MALTER. 

